Thanks, I'm in the Cayman Islands, but the KLX is a 650C (yes electric start) bought in the US (it's a standard US model), shipped to UK, driven through Europe, stored in Greece for several years, shipped to Cayman Islands. The bikes travelled way more miles that it's clocked. It's completely stock (though easy mod ideas would be appreciated).
The carb is clean... was cleaned after storage (gummed up). I beleive the petcock (thanks for the name) has a "prime" function - it has three options Off, Main, Res. I have some spare fuel line, so I'm gonna try running a line direct from the tank to the carb (bypass the petcock), to try and eliminate some variables and I'll check what fuel flow is like - thinking about it, the fuel lines did seem to be bending when I put the tank back on, so maybe they're a little too long or theres a kink or pinch somewhere - would that create this kind of issue? I'll start here and post the results this weekend.

I have a 1996 KLX 650 that's only done 10k. It's spent a lot of time in storage in various countries and last year I had it shipped to my current residence.
The bike's been running great, but in shipping the tank got dented so I pulled off the tank a month or so ago, took it to a bodywork shop, had it fixed up and refitted it (after changing spark plug). I also had to change out the gas lines between the tank and the on/off/res switch as they had gone brittle and broke off in one part.
Anyway, long story short, the bike now has a stuttering problem. It runs fine at 1/3 to 1/2 throttle but any further open and the engine starts stuttering big time...like it's going to die.
So it's most likely a problem related to something I've done (ie 1. fuel tank (could have been some crap left in it from bod shop); 2. fuel line that was replaced or 3. spark plug). I'm not a great mechanic, so any hints on the best place to start ie the most likely candidate for the problem.
thanks